The present invention relates to a new process for manufacturing aminoplastic resins. It relates more especially to a process for manufacturing urea/formaldehyde resins containing acrylic derivatives of urea.
Aminoplastic resins are known products which are widely used in the wood industry for the manufacture, in particular, of chipboard sheets. The aminoplastic resins most commonly used are urea/formaldehyde resins. They are manufactured in a known manner by the condensation of urea and formaldehyde at a pH of between 4 and 7 and at a temperature in the region of the boiling point: this condensation reaction is preferably carried out in several stages.
The main drawback of urea/formaldehyde resins is that of producing high emissions of free formaldehyde. An attempt has been made to reduce the level of free formaldehyde by employing various manufacturing processes: unfortunately, when it is desired to achieve especially low levels of formaldehyde, experience shows that this objective is accompanied both by a decrease in the reactivity and stability of the resins and also by a degradation of the mechanical properties of the finished sheets. It has also been proposed, in order to eliminate free formaldehyde, to use resins devoid of formaldehyde, in particular resins based on solutions if isocyanate. Unfortunately, the problem of formaldehyde is replaced by that of isocyanates, whose effect on man is more deleterious and longer lasting than that of formaldehyde, to the point where the presence of free isocyanate groups in chipboard sheets has been detected even several years after their manufacture.
It has also been proposed to add melamine during the manufacture of urea/formaldehyde resins. Unfortunately, the use of melamine with traditional urea/formaldehyde resins begins to be effective only at and above levels which can reach 20% and more, thereby ruling out its use on commercial grounds on account of its cost. It has thus been proposed to lower the level of melamine to values below 10%. A decrease in the reactivity of these resins is then observed, which is especially detrimental to the maintenance of commercially acceptable rates of manufacture. Moreover, even though an improvement in the properties of the finished sheets in respect of their resistance to moisture is observed, this improvement is still not sufficiently acceptable. The need hence arises to have at one's disposal urea/formaldehyde resins which, while possessing good reactivity, enable finished sheets to be manufactured possessing good mechanical characteristics linked as well with reduced levels of emission of free formaldehyde.